Fix unexpected string parsing with empty arguments

lib/nopt.js

@@ -327,6 +327,9 @@ function parse (args, data, remain, types, shorthands) {
         continue
       }
 
+      if (types[arg] === String && la === undefined)
+        la = ""
+
       if (la && la.match(/^-{2,}$/)) {
         la = undefined
         i --

package.json

@@ -5,7 +5,7 @@
   "author": "Isaac Z. Schlueter <[email protected]> (http://blog.izs.me/)",
   "main": "lib/nopt.js",
   "scripts": {
-    "test": "node lib/nopt.js"
+    "test": "node lib/nopt.js && tap test/*"
   },
   "repository": "http://github.com/isaacs/nopt",
   "bin": "./bin/nopt.js",
@@ -15,5 +15,8 @@
   },
   "dependencies": {
     "abbrev": "1"
+  },
+  "devDependencies": {
+    "tap": "~0.4.8"
   }
 }

test/basic.js

@@ -0,0 +1,16 @@
+var nopt = require("../")
+  , test = require('tap').test
+
+
+test("passing a string results in a string", function (t) {
+  var parsed = nopt({ key: String }, {}, ["--key", "myvalue"], 0)
+  t.same(parsed.key, "myvalue")
+  t.end()
+})
+
+// https://github.com/npm/nopt/issues/31
+test("Empty String results in empty string, not true", function (t) {
+  var parsed = nopt({ empty: String }, {}, ["--empty"], 0)
+  t.same(parsed.empty, "")
+  t.end()
+})